Fuel cells including a solid polymer type are expected to be used as a next-generation power generating system. A solid polymer fuel cell is particularly expected to be used as a power source for household use and automotive use, from viewpoints of a lower operating temperature and a compact shape compared to those of other fuel cells. Generally, a fuel cell is configured by laminating a plurality of cells and pressurizing and fastening the cells with a fastening member such as a bolt. One cell is configured with a membrane electrode assembly (hereinafter, referred to as an MEA) which is interposed between one pair of plate-shaped conductive separators. The membrane electrode assembly (MEA) of the solid polymer fuel cell is configured by bonding an anode, a cathode, and a polymer electrolyte membrane which is interposed between these electrodes. Fuel gas containing hydrogen is supplied to the anode through a gas diffusion layer having porosity and conductivity, oxidant gas containing oxygen such as air is supplied to the cathode through the gas diffusion layer, and the electrical power generated by an oxidation-reduction reaction occurring in each electrode is taken out. The gas diffusion layer is generally configured by providing a coating layer formed of carbon and a water-repellent material on a surface of a base material formed of carbon fibers. A mixture of a catalyst and a solid electrolyte is generally used as both reaction electrodes, in order to promote the electrochemical reaction. As the catalyst configuring the anode and the cathode, noble metals, particularly a platinum group is used as a catalyst metal, and a supported platinum catalyst in which the platinum group is supported on a carbon material such as carbon black or carbon nanofibers is broadly used. The platinum group is used as the catalyst for the fuel cells because of the activity thereof. That is, the platinum group promotes the electrode reaction of both the anode and the cathode, and has high activity.
With the spread of the fuel cells in recent years, not only excellent activity is required for the solid polymer fuel cell catalyst, but also various improvements, particularly a decrease in an amount of the platinum group used in the catalyst and a decrease in a supported amount to a support, are necessary, and various investigations have been performed.
For example, a method of stirring and mixing a solution of platinum salts and carbon powder so as to allow the platinum group to be supported on the carbon powder has been proposed (for example, see Japanese Patent No. 3643552). However, in this method, it is necessary to perform an alloying step by a high temperature treatment at 1,000° C. in the post-process, and the carbon powder which is a support may be modified due to the high temperature treatment or the supported platinum may be coalesced, and thus, it is difficult to exhibit the performance.
In addition, a method of controlling a particle diameter of a platinum group by mixing a solution of platinum group salts and a complexing agent with each other to form a platinum group complex and allow the platinum group to be supported on carbon powder has been proposed (for example, Japanese Patent No. 5524761). However, even in this method, since it is necessary to perform a thermal treatment at 650° C. to 1,000° C. in the post-process, the carbon powder which is a support may be modified due to the high temperature treatment or the supported platinum group may be coalesced, and thus, it is difficult to stably exhibit the performance. In addition, in this method, the platinum group is adsorbed onto the inside of pores of porous carbon powder to make a three-phase interface which is known as a mechanism of an electrode reaction difficult to be formed, and accordingly, the platinum catalyst is not efficiently acted in the electrode reaction.
In order to decrease a supported amount of the platinum group catalyst, without decreasing an amount of the catalyst used for the electrode reaction, it is important to provide platinum group particles supported on the carbon powder so as to have a uniform size necessary for the electrode reaction, and not to allow the platinum group catalyst to be supported on the inside of the pores of the carbon powder material which does not contribute to the electrode reaction. However, in the configuration of the related art, a size of a complex or a precursor which is a platinum source is small, and therefore the sizes thereof at the time of collection significantly vary. In addition, since the size of the pore of the carbon powder is greater than the size of the complex, the complex may be adsorbed onto the inside of pores of carbon powder, even when the complex is formed, and thus, the following problems are caused. The platinum group catalyst is aggregated to have uneven particle diameters, and the platinum group adsorbed onto the inside of pores of carbon powder is not effectively acted, and thus, activity is low.